The present invention relates to a method and apparatus for fabricating a coated optical fiber while reducing polarization mode dispersion, and to a coated optical fiber fabricated thereby.
In a conventional fabrication method of fiber drawing by heating to soften one end of an optical fiber preform and pulling a glass fiber downward therefrom, it was difficult to make the glass fiber with a core portion and a cladding portion around it in perfectly circular and concentric cross section, and their cross section was usually slightly elliptic or of slightly distorted circle shape. For this reason, the refractive index distribution in the cross section of the glass fiber wan not perfectly uniform and this caused the difference between group velocities of two polarized waves in the cross section of the glass fiber, the problem of increase in polarization mode dispersion has occurred.
Large polarization mode dispersion would raise a problem, particularly, when an optical fiber cable is used in practice as a submarine cable or a main cable required performance of which is large-capacity and long-haul transmission. A solution to this problem of polarization mode dispersion is a fabrication method of coated optical fiber, e.g., as disclosed in Japanese Patent Application Laid-open No. H9-243833, in which the glass fiber is drawn and coated to obtain a coated optical fiber and the coated optical fiber is guided by a guide roller periodically swinging the direction of its rotation axis, thereby imparting predetermined twists to the glass fiber.
FIG. 3 shows the fabrication steps of this method. An optical fiber preform 11 placed in a drawing furnace 12 is heated to soften at one end thereof and a glass fiber 13 is pulled vertically downward therefrom under drawing tension of a take-up unit 26 described hereinafter. At this time, the outside diameter of the glass fiber 13 is measured by outside diameter gauge 14 and a controller not shown controls the drawing speed, feed speed of the optical fiber preform, etc. so as to keep the fiber diameter in a prescribed range.
A coating die 15 applies an ultraviolet (UV)-curable resin 16 onto the periphery of glass fiber 13 and a UV emitting unit 17 emits UV light to cure the resin 16, thereby forming a coating. A coating die 18 further applies a UV-curable resin 19 onto the periphery of the coating and an UV emitting unit 20 emits UV light to cure this resin 19, thereby forming a second coating and obtaining a coated optical fiber 21. After that, the coated optical fiber 21 is guided via guide rollers 22, a swing guide roller 23, guide rollers 24, 25, and the take-up unit 26 to be wound onto a winding-up reel 27.
The following will explain the principle of imparting the twists to the glass fiber by the swing guide roller. FIG. 4 is a plan view of the swing guide roller. A roller rotation axis 23b of the swing guide roller 23 is always within the horizontal plane even during swinging and swings in a clockwise and a counterclockwise motion of a fixed period within an angular range of xc2x1xcex8 with respect to a reference position about a vertical axis 23c passing the center of the swing guide roller 23.
Accordingly, when the swing guide roller 23 swings from the reference position in the direction A in the figure, it goes into a state of swing guide roller 23xe2x80x2; when swinging in the opposite direction B, it goes into a state symmetric therewith with respect to the reference position, though not shown. As a result, when the swing guide roller is at the reference position, the coated optical fiber 21 descending from above is in contact with a roller surface 23a at a point Pa and travels along the roller surface so that the traveling direction thereof is bent from the vertical direction to the horizontal direction. Then, the coated optical fiber 21 travels in the direction of arrow C.
When the swing guide roller swings to move to the position of 23xe2x80x2, if the coated optical fiber 21 did not move on the roller surface 23a, the coated optical fiber 21 would first contact the roller surface 23a at a position of point Qa. However, since the coated optical fiber 21 is held under tension, the coated optical fiber 21 moves on the roller surface 23a to take the shortest course. This causes the first contact position of the coated optical fiber 21 with the roller surface 23a to move to a position of point Qb on the roller surface 23a. At this time, since friction acts between the coated optical fiber 21 and the roller surface 23a, the coated optical fiber 21 does not slide to move on the roller surface 23a, but the coated optical fiber 21 moves on the roller surface 23a while rolling about its axis. Namely, there appears the rolling motion of the coated optical fiber 21 about its axis.
When the coated optical fiber 21 rolls about its axis at the first contact position thereof with the swing guide roller 23, its rolling force is transmitted immediately above along the coated optical fiber 21 up to the softening position in the lower part of the optical fiber preform 11 from which the glass fiber is being drawn. Since the glass fiber 13 under drawing at the softening position in the lower part of the optical fiber preform 11 is still in a softening state and thus soft, the rolling force transmitted through the coated optical fiber 21 directly acts on the glass fiber 13 in the softening state at the tip of the optical fiber preform 11, so that a twist is imparted to the drawn glass fiber 13 in the softening portion at the tip of the optical fiber preform 11. Then the coatings are provided on the glass fiber 13 to make the coated optical fiber 21.
Since the optical fiber on the optical fiber preform side with respect to the swing guide roller works to transmit the rolling force generated by the rolling about the axis of the coated optical fiber at the position of the swing guide roller, mainly to the drawing portion of the glass fiber, the optical fiber itself is little subject to twisting strain between the tip of the optical fiber preform and the swing guide roller. However, the coated optical fiber 21 is twisted between the swing guide roller 23 and the guide roller 24 because of the rolling about the axis at the position of the swing guide roller 23. This twist reverses the twist direction according to inversion of the swing direction of the swing guide roller, and thus twists can be averaged in the longitudinal direction to cancel out each other. However, if the cancellation of twists in the longitudinal direction is insufficient because of existence of the guide roller and others, the residual twists will be stored in the coated optical fiber itself and move along with the travel of the coated optical fiber via the take-up unit 26 up to the winding-up reel 27.
The twists stored in the coated optical fiber are elastic torsion and thus internal stress always acts in directions to return the twists. Therefore, there will arise the problem that during a subsequent step of feeding the coated optical fiber out of the winding-up reel 27, portions of the coated optical fiber twine round each other to cause a groove state or in the worst case the coated optical fiber is forcibly drawn from the twining portions of the coated optical fiber to cause disconnection. This problem becomes more noticeable, particularly, with increase in the drawing speed in fabrication of the coated optical fiber.
The present invention provides a fabrication method and fabrication apparatus of coated optical fiber capable of relieving the elastic torsion remaining in the coated optical fiber to an unproblematic level even with increase in the drawing speed in fabrication, and also provides a coated optical fiber fabricated thereby.
The inventor had the idea that, in order to adequately cancel out the elastic torsion of the coated optical fiber in the longitudinal direction, a free zone to permit the coated optical fiber to freely rotate around the axis was provided between the swing guide roller and the winding-up reel and the coated optical fiber was allowed to twist only in this free zone, thereby reducing the residual torsion in the coated optical fiber wound up. The inventor thus conducted research to determine necessary conditions for the free zone with variations in the length of the free zone, the drawing speed in fabrication, and the number of clockwise and counterclockwise swing motions per unit time of the swing guide roller and checked the residual torsion of the coated optical fiber wound up on the winding-up reel in each of the cases. The free zone was constructed as a section in which the coated optical fiber was able to travel straight without touching any other member such as the guide roller.
The following method was used to check the residual torsion in the coated optical fiber wound up on the winding-up reel. With the coated optical fiber being wound on the winding-up reel, a mark was placed on the surface of the coated optical fiber facing to the front surface side of the winding-up reel and the coated optical fiber was fed by 1 m out of the winding-up reel to free the rotation about the axis, thereby perfectly releasing the residual, elastic torsion. Then the number of twists of the mark at that time was defined as the number of residual torsion in the coated optical fiber wound up on the winding-up reel. FIG. 5 is a diagram showing numbers of residual torsion by ∘, xcex94, and xc3x97 in a graph in which the abscissa represents the ratio of the drawing speed in fabrication to the number of clockwise and counterclockwise swing motions of the swing roller per unit time and the ordinate the zone length of the free zone. The evaluation herein was made according to the following criteria: xc3x97 for the number of residual torsion of not less than 1 twist/m, xcex94 for not less than 0.1 twist/m but less than 1 twist/m, and ∘ for less than 0.1 twist/m.
It is seen from FIG. 5 that the number of residual torsion can be controlled to below 0.1 twist/m when the length of the free zone is equal to or larger than (the drawing speed in fabrication/the number of clockwise and counterclockwise swing motions per unit time). Since the drawing speed is normally reduced immediately after a start of fabrication or immediately before an end of fabrication, it is preferable to apply a maximum drawing speed to the determination of the length L of the free zone. Letting Lo=(maximum drawing speed in fabrication)/(the number of clockwise and counterclockwise swing motions per unit time), the above inequality can be rewritten as Lxe2x89xa7Lo. From these, when the free zone is set so as to satisfy Lxe2x89xa7Lo, the residual torsion in the coated optical fiber wound up can be controlled in the preferable range of less than 0.1 twist/m.
The present invention has been accomplished on the basis of the foregoing finding and a fabrication method of coated optical fiber according to the present invention is a method of fabricating a coated optical fiber by heating to soften an end of an optical fiber preform to draw a glass fiber out therefrom, laying a coating on the glass fiber to make a coated optical fiber, and guiding the coated optical fiber via a swing guide roller periodically swinging, to twist the coated optical fiber, thereby imparting twists about the axis to the glass fiber inside the coated optical fiber, the method comprising a step of passing the coated optical fiber having passed the swing guide roller, through a free zone in which the coated optical fiber is allowed to freely rotate about the axis of the optical fiber, and thereby longitudinally canceling out elastic torsion stored in the coated optical fiber because of longitudinally alternate inversion of twist directions thereof, wherein a zone length L (m) of the free zone is not less than Lo (m) defined as follows:
Lo (m)=[a maximum drawing speed of the coated optical fiber (m/min)]/[the number of clockwise and counterclockwise swing motions per unit time of the swing guide roller (motions/min)].
In addition, a fabrication apparatus of coated optical fiber according to the present invention is an apparatus for fabricating a coated optical fiber, the apparatus comprising a drawing furnace for heating to soften an end of an optical fiber preform to draw a glass fiber out therefrom, a coating unit for laying a coating on the glass fiber, and a swing guide roller for twisting a coated optical fiber thus made with the coating, thereby imparting twists about the axis to the glass fiber, the apparatus comprising a free zone through which the coated optical fiber having passed the swing guide roller is made to pass in a free state of rotation about the axis, wherein a zone length L (m) of the free zone is not less than Lo (m) defined as follows:
Lo (m)=[a maximum drawing speed of the coated optical fiber (m/min)]/[the number of clockwise and counterclockwise swing motions per unit time of the swing guide roller (motions/min)].
In a preferable aspect, the free zone is a region between two guide members and is retained so that the coated optical fiber can travel straight between the two guide members while being maintained in a noncontact state with another member. In the free zone there may be provided at least one intermediate guide roller having a smooth roller surface for allowing the coated optical fiber to pass in a freely rotatable state about the axis. When the free zone is provided with such a guide roller having the smooth roller surface, the entire system can be constructed in relatively small size, as compared with those provided with a straight free zone.
The length of the free zone is preferably adjustable. This can be realized by locating a movable guide roller at least at one of the two ends of the free zone or by arranging at least one of intermediate guide rollers as movable. In this arrangement, the movable guide roller is moved on the occasion of hanging the coated optical fiber on the rollers to reduce the zone length L to below Lo, and after completion of the hanging, the movable guide roller is again moved to return the zone length L to the value not less than Lo, whereby the working zone for the hanging work can be shortened during the hanging of the coated optical fiber, so as to facilitate the work.